Generation and reactions of the disulphide radical anion derived from metallothionein: a pulse radiolytic study

Evidence for a Long-Lived, Cu-Coupled and Oxygen-Inert Disulfide Radical Anion in the Assembly of Metallothionein-3 Cu(I)4-Thiolate Cluster

The human copper-binding protein metallothionein-3 (MT-3) can reduce Cu(II) to Cu(I) and form a polynuclear Cu(I)₄-Cys_5-6 cluster concomitant with intramolecular disulfide bonds formation, but the cluster is unusually inert toward O₂ and redox-cycling. We utilized a combined array of rapid-mixing spectroscopic techniques to identify and characterize the transient radical intermediates formed in the reaction between Zn₇MT-3 and Cu(II) to form Cu(I)₄Zn(II)₄MT-3. Stopped-flow electronic absorption spectroscopy reveals the rapid formation of transient species with absorption centered at 430-450 nm and consistent with the generation of disulfide radical anions (DRAs) upon reduction of Cu(II) by MT-3 cysteine thiolates. These DRAs are oxygen-stable and unusually long-lived, with lifetimes in the seconds regime. Subsequent DRAs reduction by Cu(II) leads to the formation of a redox-inert Cu(I)₄-Cys₅ cluster with short Cu-Cu distances (<2.8 Å), as revealed by low-temperature (77 K) luminescence spectroscopy. Rapid freeze-quench Raman and electron paramagnetic resonance (EPR) spectroscopy characterization of the intermediates confirmed the DRA nature of the sulfur-centered radicals and their subsequent oxidation to disulfide bonds upon Cu(II) reduction, generating the final Cu(I)₄-thiolate cluster. EPR simulation analysis of the radical g- and A-values indicate that the DRAs are directly coupled to Cu(I), potentially explaining the observed DRA stability in the presence of O₂. We thus provide evidence that the MT-3 Cu(I)₄-Cys₅ cluster assembly process involves the controlled formation of novel long-lived, copper-coupled, and oxygen-stable disulfide radical anion transient intermediates.

Radiolytic modification of sulfur-containing amino acid residues in model peptides: fundamental studies for protein footprinting

Protein footprinting based on hydroxyl radical-mediated modification and quantitative mass spectroscopic analysis is a proven technique for examining protein structure, protein-ligand interactions, and structural allostery upon protein complex formation. The reactive and solvent-accessible amino acid side chains function as structural probes; however, correct structural analysis depends on the identification and quantification of all the relevant oxidative modifications within the protein sequence. Sulfur-containing amino acids are oxidized readily and the mechanisms of oxidation are particularly complex, although they have been extensively investigated by EPR and other spectroscopic methods. Here we have undertaken a detailed mass spectrometry study (using electrospray ionization mass spectrometry and tandem mass spectrometry) of model peptides containing cysteine (Cys-SH), cystine (disulfide bonded Cys), and methionine after oxidation using gamma-rays or synchrotron X-rays and have compared these results to those expected from oxidation mechanisms proposed in the literature. Radiolysis of cysteine leads to cysteine sulfonic acid (+48 Da mass shift) and cystine as the major products; other minor products including cysteine sulfinic acid (+32 Da mass shift) and serine (-16 Da mass shift) are observed. Radiolysis of cystine results in the oxidative opening of the disulfide bond and generation of cysteine sulfonic acid and sulfinic acid; however, the rate of oxidation is significantly less than that for cysteine. Radiolysis of methionine gives rise primarily to methionine sulfoxide (+16 Da mass shift); this can be further oxidized to methionine sulfone (+32 Da mass shift) or another product with a -32 Da mass shift likely due to aldehyde formation at the gamma-carbon. Due to the high reactivity of sulfur-containing amino acids, the extent of oxidation is easily influenced by secondary oxidation events or the presence of redox reagents used in standard proteolytic digestions; when these are accounted for, a reactivity order of cysteine > methionine approximately tryptophan > cystine is observed.

Mechanistic Evaluation of Arsenite Oxidation in TiO2 Assisted Photocatalysis

We report herein a detailed assessment of the roles of O2, H2O2, *OH, and O2-* in the TiO2 assisted photocatalytic oxidation (PCO) of arsenite. Although both arsenite, As(III), and arsenate, As(V), adsorb extensively onto the surface of TiO2, past studies relied primarily on the analysis of the arsenic species in solution, neglecting those adsorbed onto the surface of TiO2. We used extraction and analyses of the arsenic species adsorbed onto the surface of the TiO2 to illustrate that the oxidation of As(III) to As(V) occurs in an adsorbed state during TiO2 PCO. The TiO2 photocatalytic oxidation (PCO) of surface adsorbed As(III) in deoxygenated solutions with electron scavengers, Cu2+, and polyoxometalates (POM) yields oxidation rates that are comparable to those observed under oxygen saturation, implying the primary role of oxygen is as a scavenger of the conduction band electron. Pulse radiolysis and competition kinetics were employed to determine a rate constant of 3.6 x 10(6) M(-1) s(-1) for the reaction of As(III) with O2-*. Transient absorption studies of adsorbed hydroxyl radicals, generated by subjecting colloidal TiO2 to radiolytic conditions, provide convincing evidence that the adsorbed hydroxyl radical (TiO2+*OH) plays the central role in the oxidation with As(III) during TiO2 assisted photocatalysis. Our results suggest the reaction of superoxide anion radical does not contribute in the conversion of As(III) when compared to the reaction of As(III) with *OH radical during TiO2 PCO.

Effect of pressure on pulse radiolysis reduction of proteins

Pulse radiolysis experiments were performed on proteins under pressure. Whereas many spectroscopic techniques have shown protein modifications at different pressure ranges, the present measurements performed using the water radiolysis allowed to generate radical species and to study the mechanisms implied in their reactions with proteins. This work gives the first results obtained on the effects of pressure on the rate constants of the proteins reduction by the hydrated electron at pressures up to 100 MPa. The reaction with the hydrated electron was investigated on two classes of protein: the horse myoglobin and the mussel metallothioneins. We have successively studied the influence of the pH value of metmyoglobin solutions (pH 6, 7 and 8) and the influence of the metals nature (Zn,Cu,Cd) bound to metallothioneins. For both protein, whatever the experimental conditions, the pressure does not influence the value of the reduction rate constant in the investigated range (0.1-100 MPa).

Pulse radiolysis studies on galactose oxidase

Single-Cu-containing galactose oxidase in the GOase(semi) state (Cu(II), no Tyr(*) radical) reacts with pulse radiolysis generated formate radicals CO(2)(*-) to give an intermediate UV-vis spectrum assigned as RSSR(*-), peak at 450 nm (epsilon = 8100 M(-1) cm(-1)). From a detailed kinetic analysis at 450 nm, pH 7.0, the following steps have been identified. First the strongly reducing CO(2)(*-) (-1.9V) reduces GOase(semi) (k(0) > or = 6.5 x 10(8) M(-1) s(-1)) to a species GOase(semi)(*-). This is followed by biphasic reactions (i) GOase(semi)(*-) + GOase(semi) (k(1) = 1.6 x 10(7) M(-1) s(-1)) to give GOase(semi) + P(*-) and (ii) P(*-) + GOase(semi) (k(2) = 6.7 x 10(6) M(-1) s(-1)) to give GOase(semi)RSSR(*-). There are no significant absorbance changes for the formation of GOase(semi)(*-) and P(*-), which are Cu(I) (or related) species. However, GOase(semi)RSSR(*-) has an absorption spectrum which differs significantly from that of GOase(semi). The 450 nm peak is characteristic of an RSSR(*-) radical with two cysteines in close sequence proximity and is here assigned to Cys515-Cys518, which is at the GOase surface and 10.2 A from the Cu. On chemical modification of the RSSR group with HSPO(3)(2-) to give RSSPO(3)H(-) and RS(-), absorbance changes are approximately 50% of those previously observed. The decay of RSSR(*-) (0.17 s(-1)) results in the formation of GOase(red). No RSSR(*-) formation is observed in the reaction of GOase(semi) Tyr495Phe with CO(2)(*-), and a single process giving GOase(red)Tyr495Phe occurs. Similarly in the reaction of GOase(ox) with CO(2)(*-), a single-stage reaction gives GOase(semi).

Nitric oxide induces Zn2+ release from metallothionein by destroying zinc-sulphur clusters without concomitant formation of S-nitrosothiol

The reaction of nitric oxide (NO) with metallothionein (MT) has been investigated at neutral pH under strictly anaerobic conditions. It is observed that NO mediates zinc release from MT by destroying zinc-sulphur clusters, but that it does not by itself S-nitrosylate MT in contrast to common belief. Zinc release and loss of thiolate groups under anaerobic conditions is found to be much slower than under aerobic conditions. The observed percentage loss of Zn(2+) and thiolate groups after 3 h of NO treatment are 62 and 39%, respectively. The reaction of NO with cysteine is reinvestigated and it is found that cysteine is quantitatively converted to cystine after 5 min of NO treatment at pH 7.3. At lower pH, a much lower rate of conversion is observed confirming the base-catalysed nature of the reaction of NO with thiols. On the basis of these results, a reaction mechanism involving electrophilic attack of NO on thiolate groups and subsequent formation of a nitrogen-centred radical, MTSN(. )OH, as intermediate is proposed for the reaction of NO with MT that leads to zinc release.

Protection of DNA in HL-60 cells from damage generated by hydroxyl radicals produced by reaction of H2O2 with cell iron by zinc-metallothionein

Scavenging of hydroxyl radicals (.OH) by the zinc form of metallothionein (ZnMT) was studied in HL-60 cells and in nuclei from such cells previously treated with ZnCl2 (ZnMT cells). Cells were grown for 48 h to label DNA for alkaline elusion experiments. During the last 24 h 0.1 mM ZnMT was included to induce ZnMT. Generation of DNA single-strand breaks (SSBs) by H2O2 in cells (5 x 10(5)/ml) treated at 4 degrees was increased by approximately 70% in Zn-treated cells by comparison with control cells. These cells had grown from an initial concentration of 5 x 10(5)/ml to a concentration at harvest of 16 x 10(5)/ml. Cells started at 6 x 10(5)/ml and growing to a final concentration of 20 x 10(5)/ml did not exhibit a similar increase in SSBs. This elevation in SSBs was traced to an increase in cell Fe content which exhibited a sharp dependence upon concentrations of cells and of ZnCl2 at the time of addition. The diffusion distance (d) from Fe to DNA of ZnMT cells treated with H2O2 was found to be 3.4 nm. This compares with a distance of 6.1 nm in control cells. SSB generation by hydroxyl radicals formed by 137Cs-gamma rays in Zn-treated cells decreased by 12%, accompanied by a decrease in d from 4.8 nm to 2.9 nm. Thus, ZnMT preferentially reacts with OH formed at some distance from DNA. In nuclei isolated from ZnMT cells started at 5 x 10(5)/ml, SSB generation by H2O2 increased by 60%. The d in these nuclei was 4.9 nm, similar to the distance in control nuclei reported previously. These data suggest that, in addition to altering the scavenging environment, treatment of cells with Zn leads to an increase in reactive Fe in cells and in isolated nuclei which can generate DNA damage through reaction with H2O2.